Diffusion barriers in modified air brazes

ABSTRACT

A method for joining two ceramic parts, or a ceramic part and a metal part, and the joint formed thereby. The method provides two or more parts, a braze consisting of a mixture of copper oxide and silver, a diffusion barrier, and then heats the braze for a time and at a temperature sufficient to form the braze into a bond holding the two or more parts together. The diffusion barrier is an oxidizable metal that forms either a homogeneous component of the braze, a heterogeneous component of the braze, a separate layer bordering the braze, or combinations thereof. The oxidizable metal is selected from the group Al, Mg, Cr, Si, Ni, Co, Mn, Ti, Zr, Hf, Pt, Pd, Au, lanthanides, and combinations thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/811,633 filed Jun. 11, 2007, now U.S. Pat. No. 7,691,488 issued Apr.6, 2010.

GOVERNMENT RIGHTS STATEMENT

The invention was made with Government support under ContractDE-AC0676RLO 1830, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to methods for joining ceramic and oxidationresistant metal parts. More specifically, this invention relates toimproved braze compositions for joining ceramic and oxidation resistantmetal parts in an oxidizing atmosphere such as air.

BACKGROUND OF THE INVENTION

Joining ceramic and metal parts has proven to be one of the criticaltechnical challenges facing the material scientists fabricating devicesused in high temperature electrochemical applications. The ability tojoin a metal part to a ceramic part, or a ceramic part to anotherceramic part, theoretically provides an economical way to manufacturingcomplex ceramic components from inexpensive, simple-shaped ceramicparts, and to provide a hermetic seal between components consisting ofdissimilar materials. However, while a number of joining techniques,such as glass joining and active metal brazing are currently used, eachpossesses some form of trade-off or exhibits some penalty in terms ofjoint properties, ease of processing, and/or cost.

As an alternative, a simple and economical joining technique referred toas reactive air brazing (RAB) has been recently developed anddemonstrated for joining several different substrates. As described inJ. S. Hardy, J. Y. Kim, K. S. Weil, “Joining Mixed Conducting OxidesUsing An Air-Fired Electrically Conductive Braze,” J. Electrochem. Soc.Vol. 151, No. 8, pp. j43-j49 and U.S. patent application Ser. No.10/334,346, now U.S. Pat. No. 7,055,733 RAB differs from conventionalactive metal brazing because RAB does not require the stringentatmosphere control normally associated with conventional active metalbrazing. Instead, the RAB technique is conducted directly in air withoutthe use of flux or reducing agents to promote wetting.

The braze filler materials of the RAB consist of two ingredients, anoble metal and an oxide compound. An oxide compound, which dissolves ina molten noble metal, is added to reactively modify the oxide fayingsurface and to help the remaining molten filler material wet on it. Theresulting joint is adherent, ductile, and oxidation resistant. Due tothe ductility and compliance of the noble metal, for example silver,this brazing can be used for high temperature electrochemical devices,even though there is a significant mismatch in the coefficient ofthermal expansion (CTE) between silver (22.8 ppm/° C.) and typicalceramic components, such as yttria-stabilized zirconia (YSZ, 10.5 ppm/°C.)

One drawback of the RAB technique that has been identified insilver-copper oxide (Ag—CuO) based reactive air brazing systems for hightemperature electrochemical applications relates to the propensity ofsilver to undergo a form of high-temperature embrittlement. This occursdue to the reaction of hydrogen diffused into the braze at one side andoxygen diffused into the braze at the other side when the silver-copperoxide braze is simultaneously exposed to a reducing atmosphere on oneside and an oxidizing atmosphere on the other, as is typical in fuelcell applications. The present invention is a novel braze and method offorming a novel braze that addresses this problem, while preserving theadvantages of silver-copper oxide (Ag—CuO) based reactive air brazingsystems.

SUMMARY OF THE INVENTION

One object of this invention is to provide method for joining two parts,consisting of either two ceramic parts, or a ceramic part and a metalpart. Generally, this objective is accomplished by providing two or moreparts, providing a braze consisting of a mixture of copper oxide andsilver, providing a diffusion barrier, and heating the braze for a timeand at a temperature sufficient to form the braze into a bond holdingthe two or more parts together. Preferably, but not meant to belimiting, the copper oxide is between about 1 mol % and about 70 mol %of the silver.

Another object of this invention is to provide method for joining twoparts that forms a barrier to the diffusion of hydrogen and oxygenthrough the joint, which may lead to weakening of the joint. As usedherein, a “diffusion barrier” is thus any barrier that prevents theoxygen dissolved in the braze from reacting with hydrogen dissolved inthe braze. For example, a “diffusion barrier” may consist of alloyingelements within the braze, such as aluminum. These alloying elements maybe provided as a homogeneous component of the braze. Alternatively,these alloying elements may be provided as a heterogeneous component ofthe braze, wherein the diffusion layer is mixed with the braze at theouter edges of the braze that are exposed to oxygen and hydrogen.Further, the alloying elements may be provided as a separate layer,placed adjacent to the braze, between the braze and the oxygen andhydrogen containing atmospheres. Also, the alloying elements may becombinations of heterogeneous, homogeneous, and separate layers. In allcases, these alloying elements have higher oxygen affinity thanhydrogen, so that oxygen dissolved in the silver matrix preferentiallyreacts with these elements to form oxide rather than reacting withdissolved hydrogen.

The diffusion barrier can thus be provided as a homogeneous component ofthe braze, as a heterogeneous component of the braze, a separate layerbordering the braze, and combinations thereof. In each of theseapplications, it is preferred that the diffusion barrier be provided asan oxidizable metal. More preferred are oxidizable metals selected fromthe group Al, Mg, Cr, Si, Ni, Co, Mn, Ti, Zr, Hf, Pt, Pd, Au,lanthanides, and combinations thereof. In applications that include adiffusion layer consisting of a separate layer bordering the braze, inaddition to oxidizable metals, the diffusion barrier may also beglasses, glass ceramics, and combinations thereof.

As used herein, an “oxidizable metal” is any metal that will react withgaseous oxygen, oxygen containing gasses, or water to form the oxideform of the metal. As an example, and not to be limiting, in embodimentsof the present invention where and the braze is used to join SOFCapplications to join two components, either ceramic to metal or ceramicto ceramic, and the oxidizable metal is formed as a homogeneous and/orheterogeneous component of the braze, the oxidizable metal will reactwith oxygen in the air at one side of the joint and/or with water vaporat the opposite side of the joint, thus forming an oxide form of themetal. This oxide form then prevents the oxygen and/or hydrogen fromdiffusing into the remainder of the joint. By preventing the oxygenand/or hydrogen from diffusing into the remainder of the joint, thepresent invention prevents the oxygen and hydrogen from forming waterwithin the interior of the joint, which leads to pore formation andmechanical degradation.

While not meant to be limiting, the present invention may by used tojoin ceramics that act as insulators, and to join ceramics that act aselectrical conductors. For example, mixed ionic electronic conductingoxides such as La_(x)Sr_(1-x)FeO₃ have been shown to conduct electronsat high temperatures at about 800° C. The present invention may be usedto electrically connect or join such mixed ionic electronic conductingoxides and operate in those environments.

The braze mixture may further comprise titanium oxide. Preferably, butnot meant to be limiting, titanium oxide comprises between about 0.05mol % and 5 mol % of the braze with respect to the silver. The brazemixture may further comprise Pt, Pd and combinations thereof.Preferably, but not meant to be limiting, the Pt, Pd and combinationsthereof comprise between about 0.1 mol % and about 25 mol % with respectto the silver. The braze mixture may further comprise a ceramicparticulate filler material. Preferably, but not meant to be limiting,the ceramic particulate may comprise between about 1% and about 50% ofthe total volume of the mixture of copper oxide, silver, and ceramicparticulate. Also preferably, but not meant to be limiting, the ceramicparticulate may be smaller than 200 μm, and provided as short fibers,long fibers, powders, flakes, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the inventionwill be more readily understood when taken in conjunction with thefollowing drawing, wherein:

FIG. 1. is a binary phase diagram of Al and Ag.

FIG. 2 are low magnification cross-sectional secondary electron images(SEM micrographs) of the alumina joints brazed at 1100° C.: (a) Ag, (b)LG10, (c) LG25, and (d) LG33.

FIG. 3. are cross-sectional SEM micrographs (back-scattered images) ofthe alumina joints brazed with LG10 (9.8 at % Al): (a) 600° C., (b) 800°C., (c) 1000° C., and (d) 1100° C.

FIG. 4 are cross-sectional SEM micrographs (back-scattered images) ofthe alumina joints brazed with LG25 (26.5 at % Al): (a) 600° C., (b)800° C., (c) 1000° C., and (d) 1100° C.

FIG. 5 are cross-sectional SEM micrographs (back-scattered images) ofthe alumina joints brazed with LG33 (35.1 at % Al): (a) 600° C., (b)800° C., (c) 1000° C., and (d) 1100° C.

FIG. 6 are magnified SEM micrographs (back-scattered images) collectedfrom the braze/substrate interface of the alumina joints brazed at 1100°C.: (a) LG10 (9.8 at % Al), (b) LG25(26.5 at % Al), and (c) LG33 (35.1at % Al)

FIG. 7 are graphs showing the room temperature 4-point bend strength ofalumina joints as a function of aluminum content showing the effects of(a) Braze temperature and (b) heating rate

FIG. 8 are fracture surfaces (backscattered images) of the twocorresponding halves of fractured alumina bars joined with pure silver:(a) and (b) bars joined at 1000° C., (c) and (d) bars joined at 1100° C.

FIG. 9 are fracture surfaces (backscattered images) of the twocorresponding halves of fractured alumina bars joined with LG10 (9.8 at% Al): (a) and (b) bars joined at 1000° C., (c) and (d) bars joined at1100° C.

FIG. 10 are fracture surfaces (backscattered images) of the twocorresponding halves of fractured alumina bars joined with LG25 (26.5 at% Al): (a) and (b) bars joined at 1000° C., (c) and (d) bars joined at1100° C.

FIG. 11 are fracture surfaces (backscattered images) of the twocorresponding halves of fractured alumina bars joined with LG33 (35.1 at% Al): (a) and (b) bars joined at 1000° C., (c) and (d) bars joined at1100°

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitations of the inventivescope is thereby intended, as the scope of this invention should beevaluated with reference to the claims appended hereto. Alterations andfurther modifications in the illustrated devices, and such furtherapplications of the principles of the invention as illustrated hereinare contemplated as would normally occur to one skilled in the art towhich the invention relates.

A series of experiments were conducted to demonstrate the reduction ofone embodiment of the present invention to practice. In theseexperiments, in-situ alloying and brazing was performed using foils ofaluminum and silver. Three alloy compositions were selected based on thephase diagram shown in FIG. 1, which represent Ag (sample id # LG10),Ag₃Al (sample id # LG25), and Ag₂Al (sample id # LG33) phases. In eachof these the compositions, the sample heated up to 800° C. revealedalloying of aluminum and silver and the alloying was mostly complete at1000° C. Microstructure and mechanical properties of the joints largelydepended on alloy compositions. In the case of the braze foil with LG10(9.8 at % Al), a long continuous layer formed parallel to the directionof original aluminum foil. This indicates that aluminum was oxidizedsimultaneously while aluminum and silver diffused perpendicular to thedirection of the foils. In the bend tests, the fracture occurred throughthe long alumina/braze filler interface, resulting in low bend strength(6˜12 MPa). The joints brazed with LG25 (26.5 at % Al) showed crackspossibly due to the series of phase transformations and accompanyingabrupt volumetric changes. The fracture initiated through thesepre-existing cracks, leading to the extremely low values of jointstrength observed in these specimens. The joints prepared using LG33(35.1 at % Al) exhibited a good interface with some interfacial aluminaparticles and crack propagation through the interface between thealumina substrate and in-situ formed interfacial alumina particles ordirectly through these particles, resulting in the best bend strengthamong Al-added braze compositions.

Based on the binary Ag—Al phase diagram shown in FIG. 1, three basicAg—Al braze compositions were developed with Al contents ranging from 10to 33 at %, as shown in Table 1.

TABLE 1 Heat-treament schedule employed # of # of Sample Target Ag Al AlPhase ID (Al at %) foils foils Foil configurations* (at %) @ RT LG10 1010 1 5G/1L/5G 9.8 Ag LG25 25 9 3 3G/1L/2G/1L/2G/1L/2G 26.5 Ag₃Al LG33 338 4 2G/1L/1G/1L/2G/1L/1G/1L/2G 35.1 Ag₂Al *“G” and “L” represent silverand aluminum foils, respectively. Numbers indicate the number of 25μm-thick foils stacking together

Each composition represents one of the three major equilibrium phasesover this range: aluminum alloyed silver, Ag₂Al, and Ag₃Al. Pure silverwas used in this study as a reference baseline for mechanical propertytesting of the brazed joints. Since the inclusion of brittleintermetallic phases in the filler metals can make it difficult toproduce brazing foils by melting and rolling, each filler metalcomposition was instead prepared by in-situ alloying during the brazingprocess. This was done by laying up, in alternating fashion, foils ofsilver (Alfa Aesar, 25 μm thick, 99.95%) and aluminum (Alfa Aesar, 25 μmthick, 99.45%) of the appropriate thickness and number to achieve thetarget composition listed in Table 1.

The area specific molar ratio of Ag to Al foils was calculated byaveraging the weight out of five of each foil, all of which were cutinto the same areal dimensions (3 cm×5 cm). The molar ratio of Ag to Alper unit area of the foils was 1.081. Based on this molar ratio, thetotal number of foils was selected to give similar initial filler metalthickness while maintaining the targeted Ag/Ag ratio as close aspossible. In general the total number of Ag and Al foils was 11˜12,which yielded a foil stack thickness of approximately 265˜290 μm.

Each metal foil stack was cut into a circle measuring ˜2 cm in diameterand inserted between two alumina discs (Alfa Aesar; 99.7% purity; 2 cmin diameter×3 mm high). A dead load of ˜300 g was applied on the topdisc to ensure good contact between the stack of foils and the aluminasubstrates during the brazing process. The assemblies were heated in airat 2° C./min to a final soak temperature (600, 800, 1000, and 1100° C.)and held for 6 min before furnace-cooling to room temperature.Microstructural analysis was performed on polished cross-sections of thebrazed joints using a scanning electron microscope (SEM, JEOLJSM-5900LV), equipped with an Oxford energy dispersive X-rayspectrometer (EDS).

Room temperature 4-point bend testing was conducted to measure themechanical strength of the as-brazed joints. Bend bars were prepared byjoining the long edges of two rectangular alumina plates (Alfa Aesar;98% dense; 99.7% purity; 100 mm long×25 mm wide×4 mm thick) to form a100 mm×50 mm×4 mm plate. To keep both pieces of alumina in good contactwith the braze filler during the joining process, a dead load of 400 gwas applied to the top plate, resulting in an average pressure of ˜10kPa along the faying surfaces. Brazing was again conducted in air at ahold temperature of either 1000 or 1100° C. for 6 min. Samples wereheated to the target temperature at a rate of 2° C./min andfurnace-cooled to room temperature. To understand the effect of heatingrate on the joint strength and microstructure of these brazed specimens,samples were also heated to 1000° C. at a rate of 5° C./min. Oncejoined, each plate was machined into 4 mm×3 mm×50 mm rectangular barsfor flexural strength test. Four-point bend tests were carried out withspans between the inner and outer contact points of 20 and 40 mmrespectively at a displacement rate of 0.5 mm/min. The bend (flexural)strength was calculated from the load at failure using the standardrelationship derived for monolithic elastic materials:bend(flexural)strength=3P•L/4b•d ²

where P is the applied load, L is the length of the outer span, and band d are the respective width and height of the specimen.

Five specimens, each cut from the same plate, were used to determine theaverage room-temperature flexural strength for each joint. Scanningelectron microscopy (SEM, JEOL JSM-5900LV) was employed to examine thefracture surfaces of the specimens as means of evaluating the potentialmechanisms involved in their eventual failure.

Low magnification SEM micrographs were collected on cross sections ofalumina discs joined at 1100° C. and are shown in FIGS. 2( a)-(d). Eventhough the thicknesses of the initial braze foil stacks were similar(11-12 foils of 265-290 μm total thickness), the thickness of the fillermetal layer after brazing varied significantly and depending on thecomposition of phases formed during the brazing process. The pure silverresulted in a thin braze filler layer (˜20 μm) containing visible airpockets as seen in FIG. 2( a). At 1100° C., molten silver was squeezedout from the dead loaded joint to form molten beads on the outersurfaces of alumina plates. Along with the formation of air pockets inthe joint, this is evidence of both the low viscosity and insufficientwettability of pure silver on the alumina surface. Alternatively, jointsprepared from the aluminum-modified braze fillers (shown in FIGS. 2( b)and 1(d) display no air pockets. The joint brazed with LG10 (9.8 at %Al) exhibits a thick braze filler layer (>120 μm) and no beading of themolten braze filler, even though the brazing temperature (1100° C.) wassubstantially higher than the alloy's liquidus temperature (which isless than 950° C. and lower than the melting temperature of puresilver). This finding suggests that this filler metal composition isresistant to squeeze out (i.e. it displays good compression resistance),possibly due to a compositional dependent increase in viscosity. Jointscontaining higher aluminum content shown in FIGS. 2( c) and 2(d)exhibited similar features (no air pockets and no beading), but thinnerbraze filler layers (50˜60 μm) when compared to LG10. Since no beadingon the alumina plates was found, the Al and Agthinner braze filler layercan be attributed to the alloying of aluminum and silver, leading to theformation of intermetallic phases such as Ag₂₃Al.

The microstructure of joints prepared from the three aluminum modifiedfiller metal compositions after being heated to 600, 800, 1000 and 1100°C. are shown in FIGS. 3-5. For LG10 (9.8 at % Al), no signs ofsignificant alloying are observed when the joint heated to only 600° C.As shown in FIG. 3( a), the resulting cross-section essentially revealsthe initial configuration the stacked foils: one aluminum foil (point“b”) sandwiched between 10 silver foils (point “a” and the opposingside). The results from quantitative EDS analysis collected at each ofthe spots labeled in FIGS. 3( a)-(d) are listed in Table 2.

TABLE 2 Results of EDS quantitative analysis conducted on the spotsmarked in FIG. 3 (LG10, 9.8 at % Al). FIG. 3a (600° C.) FIG. 3b (800°C.) FIG. 3c (1000° C.) FIG. 3d (1100° C.) Element* “a” “b” “c” “d” “e”“f” “g” “h” O K — — — — — 17.43 — 38.36 Al K — 100.00 1.06 19.20 8.7335.45 7.16 46.26 Ag L 100.00 — 98.94 80.80 91.27 47.11 92.84 15.38 *Allcompositions listed are in at %.

The local chemistries measured at points “a” and “b” indicate that nomeasurable alloying takes place in the LG10 material at 600° C. However,the foils appear to be well bonded together despite this lack ofchemical interaction. At 800° C., obvious alloying between the Al and Agtakes place, accompanied by shrinkage of the filler metal thickness asseen in FIG. 3( b). However alloying remains incomplete as indicated bythe local chemistries measured at point “c” and “d”, each of whichrespectively marks the initial sites for the silver and aluminum foils.In addition, there is no indication that extensive oxidation occurs(despite the fact that brazing was conducted in air) or that bondingtakes place between the filler metal and the alumina substrate.

As shown in FIG. 3( c) and Table 2, the joint brazed at 1000° C.displays a more homogeneous distribution of aluminum within the fillermetal, with distinct regions of alumina formed parallel to the originalaluminum foil direction (e.g. point “f”).

TABLE 3 Results of EDS quantitative analysis conducted on the spotsmarked in FIG. 4 (LG25, 26.5 at % Al). FIG. 4a FIG. 4b FIG. 4c FIG. 4d(600° C.) (800° C.) (1000° C.) (1100° C.) Element* “a” “b” “c” “d” “e”“f” “g” C K — — — — — 77.35 — O K — — — — — 5.82 — Al K 2.50 99.77 24.642.82 23.82 8.43 21.59 Ag L 97.50 0.23 75.36 97.18 76.18 8.39 78.41 *Allcompositions listed are in at %.

EDS analysis conducted at point “e” near the braze/substrate interfacereveals 8.73 at % Al, which is quite close to original targetcomposition for this filler metal (9.8 at % Al)

Good bonding between the braze filler and the alumina substrate wasobserved as indicated by the penetration of molten braze into the roughsurface of the alumina substrate. Even after brazing at the highestbrazing temperature of 1100° C. (shown in FIG. 3( d)), the majority ofthe aluminum still remains in metallic form alloyed with the silvermatrix (point “g”: 7.16 at % Al) even though it is apparent that moreextensive oxidation has occurred at this temperature (see point “h”)than at the lower brazing temperatures.

The filler metal composed of 26.5 at % Al (LG25) exhibited a similartemperature dependent alloying process, as seen in the sequence ofmicrographs shown in FIGS. 4( a)-(d). No significant interaction betweenthe Al and Ag foils occurs at 600° C., which displays the original foilstacking arrangement shown in FIG. 4( a).

TABLE 4 Results of EDS quantitative analysis conducted on the spotsmarked in FIG. 5 (LG33, 35.1 at % Al). FIG. 5a FIG. 5b FIG. 5c. FIG. 5d(600° C.) (800° C.) (1000° C.) (1100° C.) Element* “a” “b” “c” “d” “e”“f” “g” O K — — — — — 48.18 — Al K 1.91 99.61 32.33 1.66 30.92 44.5230.23 Ag L 98.09 0.39 67.67 98.34 69.08 7.30 69.77 *All compositionslisted are in at %.

Alloying is observed upon brazing at 800° C. as shown in FIG. 4( b). Themore extensive alloying of this braze composition at 800° C., comparedto LG10, is attributed to the lower liquidus temperature of thiscomposition as well as the thinner silver foils employed in preparingthis filler metal. However, the EDS results given in Table 3 indicatesome inhomogeneity in the filler metal matrix. While the matrixrepresented by spot “c” contains 24.64 at % Al, which is close to theinitial Al content in the braze foil stack, silver-rich particles arealso found in the matrix (e.g. point “d”, which displays only 2.82 at %Al). An acceptable interface between the braze filler and the substrateis observed, as shown in FIG. 4( c), when the joint is brazed at 1000°C. The matrix phase (point “e”) exhibits improved homogeneity, althoughthe silver-rich phase is still observed, predominantly at thebraze/substrate interface.

A distinctive microstructural feature observed in this joint is thecrack found between the filler metal and substrate indicated by point“f”. Cracking due to embrittlement is possibly related to the complexseries of phase transformations that this composition likely undergoesduring cooling, as observed in the phase equilibrium diagram of FIG. 1(i.e. liquid→Ag+liquid→Ag+β-Ag₃Al→Ag→Ag+α-Ag₃Al). The joint brazed at1100° C. shown in FIG. 4( d) also exhibits cracks, as well as extensiveformation of alumina in particulate form. Despite this degree ofoxidation, the majority of aluminum still remains in the metallic matrixphase shown at point “g” in FIG. 4( d): 21.59 at % Al.

FIG. 5 shows the microstructures of joints brazed using the LG33 fillermetal (35.1 at % Al) at the four different soak temperatures. Similar toLG25, extensive alloying is observed in the entire braze filler layer at800° C. as shown in FIG. 5( b), while no significant interaction betweenAg and Al is detected at 600° C. as shown in FIG. 5( a). No significantoxidation of aluminum is observed in the specimen prepared at 800° C.The matrix phase (point “c”) contains 32.33 at % Al (as indicated inTable 4), while a silver-rich phase observed along the fillermetal/substrate interface displays only 1.66 at % Al. As shown in FIG.5( c), the matrix phase (at point “e”) formed at 1000° C. still contains30.92 at % Al even though some alumina formation is observed in thebraze filler as well as along the braze/substrate interface. Poorbonding between the braze/substrate interface is observed on the rightside of the joint, while the interface on the other side looksacceptable. Massive oxide formation on the de-bonded interface (at point“f”) implies that poor contact between the braze filler and thesubstrate may cause oxidation of the braze filler surface before thebraze melt wets the ceramic substrate, leading to reduced interfacialbonding. The joint brazed at 1100° C., shown in FIG. 5( d), stillcontains a majority of Al in the braze matrix (point “g” in Table 4)even though extensive oxide formation takes place in the bulk fillermetal, as well as along the interface.

FIG. 6 shows magnified SEM micrographs collected on the fillermetal/substrate interfaces of specimens brazed with each of theAl-modified filler metal compositions at 1100° C. All of the resultingfiller metal compositions exhibit good interfacial bonding due towetting of the molten braze filler on the substrate. Additionally the LG33 material (containing the highest aluminum content; 35.1 at % Al)displays interfacial oxide formation along the braze/substrateinterface.

FIGS. 7( a) and (b) are graphs showing two plots of room temperatureflexural strength as a function of aluminum content. FIG. 7( a) displaysthe effect of the final soak temperature on bend strength, while FIG. 7(b) shows the effect of heating rate. As seen in FIG. 7( a), there is nosignificant difference in bend strength between the joints brazed at1000° C. and 1100° C. even though more extensive formation of aluminawas observed at 1100° C. The bars joined with pure silver exhibitaverage bend strength of 71 MPa for the sample brazed at 1000° C. and 79MPa for the sample brazed at 1100° C. However, the LG 10 (9.8 at % Al)specimens display poor bend strength, 6 MPa after brazing at 1000° C.and 12 MPa at 1100° C. In the case of the LG25 (26.5 at % Al) specimens,the resulting joints were so weak that fracture often took place duringsample preparation. The poor bend strength of the LG10 and LG25 jointswas unexpected, particularly given that SEM examination revealed adecent filler metal/substrate interface in each. The bend bars brazedwith LG 33 (35.1 at % Al) exhibit bend strengths of 46 MPa (1000° C.soak temperature) and 52 MPa (1100° C. soak temperature), comparablewith pure silver. FIG. 7( b) shows the effect of heating rate on themechanical properties of joints. The higher heating rate of 5° C./mingenerally shows no improvement in bend strength compared to slowerheating rate of 2° C./min, particularly at the low aluminum containingfiller metal compositions. This result corresponds to the evidence foundin the SEM and EDS analyses since most of the Al remains in metallicform in the silver matrix phase and there were no apparent differencesobserved between the filler metal/substrate interfaces in thesespecimens. Therefore rapid heating rate, which can reduce the formationof alumina, may not significantly improve the filler metal/substrateinterface.

To better understand the mode of failure in these joints, SEM analysiswas conducted on the fractured surfaces of the bend specimens. FIGS.8-11 are back-scattered SEM images of comparative sets of fracturedjoining specimens that were brazed with different filler metalcompositions at 1000° C. and 1100° C. FIGS. 8( a) and 8(b) are the twofractured halves of specimen brazed with pure silver at 1000° C., anddisplay cup-cone marking dimples that are indicative of ductilefracture. In these samples, joint failure occurred within the bulk ofthe joint rather than at the interfaces or within the aluminasubstrates, which further suggests that good adhesion exists between thefiller metal and the substrate.

The fracture surfaces of the pure silver specimen brazed at 1100° C.also exhibit similar signs of ductile as shown in FIGS. 8( c) and 8(d).The corresponding halves of the fractured LG10 specimen brazed at 1000°C. are shown in FIGS. 9( a) and 9(b). Unlike pure silver, these twosurfaces display a thin alumina layer (dark phase) on a relativelysmooth Ag—Al matrix surface (white). Since the morphology of the in-situformed alumina is distinctively different from that of aluminasubstrate, the thin alumina observed is attributed to an in-situ layerformed in the filler metal, as shown in FIG. 3( c). The fracture surfaceof this specimen thus indicates that failure occurred through thein-situ alumina layer in the filler metal, and not along thebraze/substrate interface. This is why this particular filler metalexhibits low bend strength despite forming a good interface with thealumina substrate.

In order to improve the strength of this filler metal, the in-situalumina must form in a more localized manner as separate particles withsufficient soft matrix in between, rather than as well aligned brittlelayers. This could be achieved by using a pre-alloyed braze foil, ratherthan an in-situ alloyed material. The bar brazed at 1100° C., shown inFIGS. 9( c) and 9(d), exhibits the same mechanism of fracture, althoughthe alumina layers are more obviously apparent due to the greater extentof oxide formation in this higher temperature specimen.

The fractured surfaces of the LG25 bend bar specimens are shown in FIG.10. Both of the bars joined at 1000° C. as shown in FIGS. 10( a) and10(b), and 1100° C. as shown in FIGS. 10( c) and 10(d), displaypre-fracture cracks, which were also observed in the correspondingcross-sectional micrographs shown in FIG. 4. The fracture initiatedthrough these pre-existing cracks, leading to the extremely low valuesof joint strength observed in these specimens. As discussed previously,it is suspected that the existence of these flaws is due to the seriesof phase transformations (and accompanying abrupt volumetric changes)that occur in this material upon cooling form the molten state.

As shown in FIG. 11, the bend bar specimens prepared using LG33 (35.1 at% Al) exhibit a substantially different fracture surface. One of thesurfaces in the bar brazed at 1000° C., shown in FIG. 11( a), displaysfiller metal covered with fine alumina particles measuring less than 5μm in size. The corresponding half displays essentially a clean surfaceof the alumina substrate (grain size around 10 μm) with some smalleralumina particles. The smaller particles can be attributed tointerfacial alumina that forms during the brazing process. Thus, crackpropagation appears to take place through the interface between thealumina substrate and in-situ formed interfacial alumina particles ordirectly through these particles. Since fracture occurred at or nearthis interface and this joint displays a good interface as shown in FIG.6( c), the best bend strength among Al-added braze compositions wasachieved using this filler metal composition.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character. Only certain embodimentshave been shown and described, and all changes, equivalents, andmodifications that come within the spirit of the invention describedherein are desired to be protected. Any experiments, experimentalexamples, or experimental results provided herein are intended to beillustrative of the present invention and should not be consideredlimiting or restrictive with regard to the invention scope. Further, anytheory, mechanism of operation, proof, or finding stated herein is meantto further enhance understanding of the present invention and is notintended to limit the present invention in any way to such theory,mechanism of operation, proof, or finding.

Thus, the specifics of this description and the attached drawings shouldnot be interpreted to limit the scope of this invention to the specificsthereof. Rather, the scope of this invention should be evaluated withreference to the claims appended hereto. In reading the claims it isintended that when words such as “a”, “an”, “at least one”, and “atleast a portion” are used there is no intention to limit the claims toonly one item unless specifically stated to the contrary in the claims.Further, when the language “at least a portion” and/or “a portion” isused, the claims may include a portion and/or the entire items unlessspecifically stated to the contrary. Likewise, where the term “input” or“output” is used in connection with an electric device or fluidprocessing unit, it should be understood to comprehend singular orplural and one or more signal channels or fluid lines as appropriate inthe context. Finally, all publications, patents, and patent applicationscited in this specification are herein incorporated by reference to theextent not inconsistent with the present disclosure as if each werespecifically and individually indicated to be incorporated by referenceand set forth in its entirety herein.

We claim:
 1. A new method for joining two parts comprising the steps ofproviding two or more parts, at least one of the parts comprising aceramic material; providing a braze comprising copper oxide, silver,dissolved oxygen, dissolved hydrogen, and a diffusion barrier; whereinthe diffusion barrier is one or more of a homogeneous component of thebraze, a heterogeneous component of the braze, and/or a separate layerbordering the braze; preventing the oxygen dissolved in the braze fromreacting with the hydrogen dissolved in the braze using the diffusionbarrier, and placing the braze in between the two parts; and heatingsaid braze for a time and at a temperature sufficient to form the brazeinto a bond holding the two or more parts together.
 2. The method ofclaim 1 wherein the homogeneous component of the braze comprises one ormore of Al, Mg, Cr, Si, Ni, Co, Mn, Ti, Zr, Hf, Pt, Pd, Au, and/orlanthanides.
 3. The method of claim 1 wherein the heterogeneouscomponent of the braze comprises one or more of Al, Mg, Cr, Si, Ni, Co,Mn, Ti, Zr, Hf, Pt, Pd, Au, and/or lanthanides.
 4. The method of claim 1wherein the separate layer bordering the braze comprises one or more ofAl, Mg, Cr, Si, Ni, Co, Mn, Ti, Zr, Hf, Pt, Pd, Au, and/or lanthanides.5. The method of claim 1 wherein the separate layer bordering the brazecomprises one or both of glasses and/or glass ceramics.
 6. The method ofclaim 1 wherein the amount of copper oxide in the braze is between about1 mol % and about 70 mol % of the amount of silver in the braze.
 7. Themethod of claim 1 wherein at least two of the bonded parts and the bondare electrically conductive.
 8. The method of claim 1 wherein thehomogeneous component of the braze is an oxidizable metal.
 9. The methodof claim 1 wherein the heterogeneous component of the braze is anoxidizable metal.
 10. The method of claim 1 wherein the separate layerbordering the braze is an oxidizable metal.
 11. The method of claim 1wherein the braze further comprises titanium oxide.
 12. The method ofclaim 11 wherein the amount of titanium oxide in the braze is betweenabout 0.05 mol % and 5 mol % of the amount of the silver in the braze.13. The method of claim 1 wherein the braze further comprises Pt and/orPd.
 14. The method of claim 13 wherein the amount of the Pt and/or Pd inthe braze is between about 0.1 mol % and about 25 mol % of the amount ofsilver in the braze.
 15. The method of claim 1 wherein the braze furthercomprises a ceramic particulate filler material.
 16. The method of claim15 wherein the amount of the ceramic particulate in the braze is betweenabout 1% and about 50% of the total volume of the copper oxide, thesilver, and the ceramic particulate in the braze.
 17. The method ofclaim 15 wherein the ceramic particulate is smaller than 200 μum. 18.The method of claim 15 wherein the ceramic particulate comprises one ormore of short fibers, long fibers, powders, and/or flakes.
 19. Themethod of claim 1 wherein the braze further comprises one or more oftitanium oxide, Pt, Pd, and/or ceramic particulate filler material.